This application relates to devices that improve the aerodynamic performance of vehicles and more particularly to air flow control surfaces on trucks.
Over-the-highway trucks and tractor-trailer combinations come in many different sizes and configurations, and the aerodynamic properties of those configurations can have a significant effect on the operational efficiency of the vehicles. Accordingly, fairings and other devices for improving the aerodynamic performance of load-carrying vehicles have been developed.
For example, U.S. Pat. No. 4,245,862 to Buckley, Jr., describes a drag-reducing device mounted on the roof of the cab of a vehicle that includes a trailing body for reducing aerodynamic drag. The patent states that a substantial part of the aerodynamic drag experienced by a tractor-trailer combination moving over the highway is the result of air flow separations that occur at the forward edges of the trailer. In one embodiment, a trim tab is hinged along a fairing's back edge, which is horizontal, and is inclined upward at an angle into the air flow above the fairing, deflecting the air flow upward.
U.S. Pat. No. 4,290,639 to Herpel describes a wind deflector having a movable fairing that is mounted on the tractor cab of a tractor-trailer combination to allow for the reduction of wind resistance during movement of the vehicle. The deflector fairing pivots between operative positions, in which air is directed to reduce wind resistance, and a rest position, in which the fairing has no substantial effect on wind resistance.
U.S. Pat. No. 4,006,931 to Groves describes a device for reducing the wind resistance of a vehicle. A false roof of generally inverted-U shape is positioned on top of the roof of the vehicle's cab to form an air-flow channel, and an adjustable vane is positioned at the rear of the channel to direct air flowing through the channel upwardly and over the roof of the trailer. The inclination of the vane is adjustable to accommodate trailers having different heights.
These patents describe devices that are placed in the air flow around a vehicle and operate as air dams, pushing the flow into new directions.
As noted above, the relative geometry of the tractor and trailer in a load-carrying vehicle such as a class-8 tractor-trailer combination can vary widely, and these variations can affect the relative height and length of the gap between the tractor and the trailer and thus the aerodynamic performance of the combination vehicle.
The roof of the cab of a tractor or truck can sit at various heights above the ground, depending on tire diameter, suspension geometry, chassis height, and type of cab, among other factors. Trailers are also commercially available with various container heights, suspensions, and tire choices. In addition, “fifth wheels”, which couple tractors to trailers, have various heights above the ground and also affect the relative heights of the cab and trailer and the aerodynamic relationship between them.
Another variable factor in trucks is the length of the gap between the back of a cab and the front of a trailer. The gap varies based on vehicle loading, driver preferences for ride, the position of the fifth wheel with respect to the cab, and the length of the trailer overhanging in front of the fifth wheel, among other factors. Some of these factors can be complicated because some tractors have sliding fifth wheels, which can be used to manage vehicle weight distributions by moving the trailer closer to or farther from the tractor, thereby changing the gap length.
Varying relative heights and gap lengths between cabs and trailers produce varying aerodynamic performance. The shape of the exterior surfaces of a truck is usually optimized for aerodynamic performance despite these variations, but the best performance may be obtained only for a specific combination of relative height and gap length, based on expected high volume market demands for a specific vehicle configuration, for example, a specific relative height and a short gap. As a combination diverges from the optimum height and gap combination, the aerodynamic performance also diverges from the peak performance.